错误日志：
There is no Action mapped for namespace / and action name HelloWorld. - [unknown location]
	com.opensymphony.xwork2.DefaultActionProxy.prepare(DefaultActionProxy.java:186)
	org.apache.struts2.impl.StrutsActionProxyFactory.createActionProxy(StrutsActionProxyFactory.java:41)
	org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:494)
	org.apache.struts2.dispatcher.FilterDispatcher.doFilter(FilterDispatcher.java:419)

解决：
 将struts.xml文件放到/WEB-INF/classes下面，解决问题。
struts.xml文件要包含：<!DOCTYPE struts PUBLIC
    "-//Apache Software Foundation//DTD Struts Configuration 2.0//EN"
    "http://struts.apache.org/dtds/struts-2.0.dtd">这部分声明。